Crystalline thermoplastic polyester resins, such as polyalkylene terephthalate resins, have been used extensively as an engineering plastic in various fields owing to their excellent mechanical and electrical properties, as well as their physical and chemical characteristics and relative processing ease. Thus, thermoplastic polyester resins have been used to form molded parts including automotive, electrical and electronic applications.
Diversifying the end-use applications for thermoplastic polyester resins, however, often requires specific performance and/or property characteristics. For example, it is desirable for parts used in the automotive industry to exhibit specific mechanical properties in terms of low temperature flexibility and impact resistance so as to meet automotive safety standards.
Polyester resins have previously been blended with a thermoplastic elastomeric resin, such as an olefin polymer, or a relatively more "rubbery" polymer in an attempt to obtain satisfactory low temperature properties. Although polyester resins blended with an elastomeric resin typically will exhibit some improvement in low temperature properties, the resulting polyblend composition is highly susceptible to surface peeling due to poor compatibility between the polyester and elastomeric resin components.
Furthermore, polyester resins are known generally for their excellent processability and moldabiliy. However, the coexistence of an elastomeric resin with a polyester base resin tends to deteriorate the mold-release properties of the composition. Decreased mold-release properties (as compared to polyester resins generally) may lead to deformation, cracking and the like of articles molded from the resin. These problems are especially acute when articles having a relatively complex shape, small size and/or small wall thickness ar attempted to be molded.
In order to counteract the moldabiliy problems encountered with blends of polyester and elastomeric resins, paraffin oils, fatty acid amides, fatty acid esters and the like have been melt-blended with polyester compositions as mold-release agents. Polyester resins which include such conventional mold-release agents do exhibit improvements in their mold release properties, but typically will exhibit poor extruder screw "bite" (and thus slippage on the extruder screw) during injection molding due to blooming of the mold release agents which occurs during a preliminary pellet drying stage. As a result, insufficient plasticization of the resin composition requiring additional time for the screw extrusion stage occurs. As a result, molded articles of consistent quality can typically not be produced over a period of time.
Recycling of resin scrap has, in recent years, gained significant importance not only from an environmental point of view, but also form an economic point of view for resin manufacturers and molding operators. Reuse of scrap resin, however, only exacerbates the poor extruder screw "bite" characteristics of the virgin resin which include conventional mold-release agents. While the problems of poor extruder bite can sometimes be ameliorated to a certain extent by the application of a metallic soap to the surface of resin pellets prior to the extrusion operation, such a procedure is necessarily more costly since an additional process step is required. Furthermore, metallic soaps are corrosive requiring more frequent equipment maintenance and/or replacement, and can also be an environmental problem within the molding plant requiring complex (and costly) recovery schemes.
What has been needed in the art, therefore, is a polyester molding compositions that not only exhibits exceptional low temperature physical properties (e.g., flexibility and toughness), but also has excellent mold-release properties so that articles of relatively complex shape, small size and/or small wall thickness can be made. It is towards satisfying such a need that the present invention is directed.
The present invention is embodied in polyester molding compositions which exhibit excellent low-temperature properties, especially low-temperature toughness, and improved moldabiliy, especially mold-release properties. Pellets formed of the compositions of this invention furthermore exhibit satisfactory extruder screw "bite" during prolonged injection molding operations so that consistent molded articles of complex shapes, small sizes and/or small wall thickness can be made.
The compositions of the present invention are broadly characterized by a melt-blend of a crystalline polyester base resin, a polyester elastomer and a sorbitan ester. More specifically, the compositions of this invention are characterized by a melt-blend of (A) a crystalline thermoplastic polyester base resin, (B) between 1 to 50 parts by weight, based on the weight of the polyester base resin, of a polyester elastomer, and (C) a mold-release effective amount of between about 0.01 to 10 parts by weight, based on the total composition weight, of a sorbitan ester which is the reaction product of a sorbitan with at least a 12 carbon atom fatty acid. Most preferably, the sorbitan ester mold-release agent has a hydroxyl value of between 50 and 400 so that blooming during predrying stages of an injection molding operation is minimized (if not eliminated entirely).
Further aspects and advantages of this invention will become more clear after careful consideration is given to the detailed description of the preferred exemplary embodiments thereof which follow.